1. Field of the Invention
The present invention relates to a laser light source apparatus and a method of controlling the laser light source apparatus and, more particularly, to a laser light source apparatus and method suitable for optomagnetic recording.
2. Description of the Related Art
A laser light source apparatus is known which causes second harmonic oscillation (SHG) of, for example, green laser light or fourth harmonic oscillation (FHG) of, for example, ultraviolet laser light (referred to as UV laser, hereinafter) by wavelength-converting long-wavelength laser light with a wavelength conversion element formed of a nonlinear optical crystal.
FIG. 1 is a block diagram schematically showing the configuration of a laser light source apparatus of this kind. The laser light source apparatus has a green resonator unit 1, in which long-wavelength laser light is introduced into a wavelength conversion element formed of a nonlinear optical crystal, and green laser light G is output by resonance of the introduced laser light with the wavelength conversion element. The green resonator unit 1 has constituents 1a to 1d and 1f.
The constituent 1a is an exciting laser device for emitting exciting laser light. The constituent 1b is an exciting laser control section for controlling the light emitting operation of the exciting laser device 1a. The constituent 1c is a green resonator consisting of a light emitting element for emitting YAG laser light according to the exciting laser light, a nonlinear optical KTP crystal element (hereinafter referred to as "KTP crystal") for wavelength-converting the YAG laser light, and a resonance optical system.
The constituents 1d and 1f are temperature control sections for controlling the resonance optical system and the KTP crystal.
The laser light source apparatus also has an UV resonator unit 2 for causing resonance of the green laser light emitted from the above-described green resonator section 1 to cause fourth harmonic oscillation (FHG) of UV laser light. The UV resonator unit 2 has a phase modulator 2a, a green detector 2b, a locking control section 2c, a BBO (.beta.-BaB.sub.2 O4) element 2d, an acousto-optic modulator (AOM) 2e, an AOM control section 2f and an optical element such as a half mirror.
In order to obtain high-power stable UV laser light by the laser light source apparatus arranged as described above, it is necessary to maintain the KTP (KTiPO.sub.4) crystal at an optimal operating temperature.
The KTP crystal has such a temperature characteristic that the UV laser light output changes as the crystal temperature changes from 42.4.degree. C. to 43.1.degree. C., as shown in FIG. 2, for example. In such a case, an optimal temperature of 42.8.degree. C. exists, at which the output is maximized.
Conventionally, the KTP temperature control section if performs a feedback temperature control for maintaining such an optimal crystal temperature. That is, as shown in FIG. 3, a reference potential REF is applied to the noninverting input terminal of an error amplifier 10 formed of an operational amplifier or the like while the output from a thermistor 13 described below is supplied to the inverting input terminal of the error amplifier 10. The error amplifier 10 generates an output signal according to the potential difference between the reference potential REF and the output of the thermistor 13.
The output driver 11 generates a drive signal for cool-driving or heat-driving a Peltier element 12 according to the output of the error amplifier 10. The Peltier element 12 electronically cools or heats the KTP crystal according to the drive signal generated by the output driver 11. The resistance of the thermistor 13 changes according to the temperature of the KTP crystal. From the thermistor 13, the output voltage corresponding to a measured value of the KTP crystal temperature is supplied to the inverting input terminal of the error amplifier 10.
If the reference potential REF is set to the value corresponding to the target temperature, the KTP temperature control section 1f performs feedback control of making the Peltier element 12 electronically cool and heat the KTP crystal so that the KTP crystal is maintained at the target temperature.
The above-described conventional laser light source apparatus entails problems described below.
1) The KTP temperature control section 1f performs feedback temperature control such as to constantly maintain the KTP crystal at a certain temperature. However, it is known that the optimal temperature of the KTP crystal is not a fixed value; it varies depending upon environmental conditions, such as room temperature and humidity, and upon the temperature characteristic of the KTP crystal, which is not stable. The conventional KTP temperature control section 1f cannot follow up such variation in optimal temperature, so that the UV laser output is unstable.
2) As can be understood from the temperature characteristic of the KTP crystal shown in FIG. 2, the KTP crystal temperature must be changed a 0.1.degree. or smaller step to detect the optimal temperature. Moreover, this is done manually according to the conventional art. Such a manual operation is very difficult to perform.
3) In the conventional laser light source apparatus, the operations of the exciting laser control section 1b, the resonator temperature control section 1d, the locking control section 2c and the AOM control section 2f are respectively controlled as well as that of the KTP temperature control section 1f. Therefore, there is a need for efficient optimal control of the entire light source system.